1. Field of the Invention
The present invention relates to a filler for paper-making and a neutral paper-making process by the use thereof. More particularly it relates to a filler composition which is cheap and extremely good with regard to abrasive wear of plastic wire and to a paper-making process by the use thereof.
2. Description of the Prior Art
There are two alternative kinds of paper-making process according to the kind of sizing agent etc. used, namely the acid process and the neutral process.
These two paper-making processes are distinguished from each other by the sizing agent, fixing agent and filler used. There are advantages as well as disadvantages in the quality of the paper made, but so far the greater part is practiced by the acid process by reason of the total cost as well as technical problems, the neutral paper-making process being used only partly.
In the acid paper-making process rosin is used as sizing agent and aluminum sulfate (normally called alum) as fixing agent. This is an economically as well as technically advantageous process in that both rosin and aluminum sulfate are cheap and aluminum sulfate plays an important role throughout the paper-making process, being not only effective for fixing of the rosin size but also effective for prevention of slime formation and pitch deposition in the paper-making equipment. Further, in this acid paper-making process characteristic is the use of filler. As filler kaolin, clay, titanium dioxide etc. are used, and aluminum sulfate contributes to improvement of the yield of these fillers. These fillers are, however, economically problematic, good talc, for instance, being difficult to obtain in Japan and titanium dioxide being inevitably expensive for its complicated process of manufacture. Moreover, since the paper made by the acid paper-making process is acidic, it is bound to be humid, and with the resultant increase of the paper's water content, increased is the amount of hydrogen ion so that the carbohydrate becomes subject to oxidation and hydrolysis, while it is also subject to decomposition and modification by other ions, this resulting in the paper's increased rate of deterioration, and for this reason papers made by the acid paper-making process have been accepted to be less suited for use where stability in prolonged storage is essential such as paper for books e.g. encyclopoedia and official documents. Also, since in the case of acid paper-making the paper-making process is conducted normally in a pH range of 4-5, the paper-making machine and other metal parts are highly subject to corrosion.
Neutral paper-making process, on the other hand, is a process in which a neutral sizing agent of the type alkyl ketene dimer or alkyl succinic acid, cationic fixing agent and emulsifier are used so that the pH of white water, the material for paper-making, is adjusted to not less than 7, also called the "alkaline paper-making process."
Calcium carbonate has been known as being equally effective compared with the above-mentioned filler with regard to its effect to improve paper's whiteness, opacity, smoothness, affinity for ink, writing property and coating property etc. Although it is available domestically amply and cheaply, it cannot be used in the acid paper-making process as it enters into a chemical reaction with aluminum sulfate. In this neutral paper-making process, however, calcium carbonate can be used, and actually Indian paper, rice paper and back-carbon paper etc. are being manufactured with calcium carbonate as filler. Adoption of this neutral paper-making process not only means improvement of paper quality such as improvement in whiteness and intransparency and prevention of deterioration of the paper quality but also cost merit due to increase of the paper's ash content, for equally good paper quality is attainable even when the filler content is increased 33-5% compared with acid-manufactured paper. Other advantages are decrease of beating power, improvement of reutilization of white water due to decrease of separating salts, decrease in amount of steam required for heating fresh water due to improvement of white water reutilization rate, decrease of specific energy consumption due to decrease of depositing lees of wet parts etc. attributable to decrease of separating salts and also operative advantage. Furthermore, in the field of paper coating, higher concentration of coating color has been sought for improvement of the physical properties of coated paper and lowering of the energy cost, and as coating material heavy calcium carbonate in fine particle size is now being used progressively in combination with a lot of kaolin. When the waste portion of such coated paper having coated thereon a high specific amount of heavy calcium carbonate (waste from the paper-making process, finishing process etc.) is reclaimed by the acid paper-making process, calcium carbonate enters into chemical reaction with the acid contained in aluminum sulfate to cause foaming due to generation and separation of carbon dioxide, hence inside filler size treatment by rosin-aluminum sulfate system becomes difficult or there results sedimentationsolidification of the resulting calcium sulfate etc. in the vessel or pipe interior to possibly induce unexpected trouble in the fluid forwarding process and other operative steps.
For the above-mentioned steps a switchover from the acid paper-making process to the neutral counterpart is desired. The neutral sizing agent used for this neutral paper-making process is still extremely expensive compared with rosin-aluminum sulfate despite the studies from various sides for reducing its cost, this being the only cost-raising factor for the neutral paper-making process which is aimed at cost-saving, and switchover to neutral paper-making process is difficult unless an effort is made in earnest for saving of the total cost by the use of a filler as cheap as possible. If cheap calcium carbonate is used as filler, there arises a problem of abrasive wear of wires of the paper-making machine as stated below.
In recent years, with the paper-making machine increasing in scale as well as in operating speed, there has been a growing interest in wire life from the viewpoint of productivity as well as workability. To cope with the growing interest, plastic wire was developed, and put to practical use beginning since late in 1960s, and today plastic wire has been almost standardized for large-scale machines, and it is now being used also for medium-small machines. The reason for the progressive adoption of plastic wire in place of bronze wire is for plastic wire's advantage over bronze wire with regard to fatigue life, safety from corrosion and easiness required for maintenance. Moreover, the abrasive effect of talc and kaolin mainly used as filler for the acid paper-making process is much less on plastic wire than on bronze wire, the life of the plastic wire in the process thus being 5-10 times longer than that of the bronze wire. Things are, however, different if calcium carbonate is used as filler instead of talc, kaolin etc. So, when neutral paper-making is carried out on a paper-making machine with calcium carbonate as filler, calcium carbonate's abrasive wear of plastic wires is different from that of talc or kaolin, and its abrasive effect on plastic wires is higher than on bronze wires, this resulting in a marked adverse effect on working efficiency of the neutral paper-making process. Since badly abrased wire has to be replaced with a new one with the paper-making machine being stopped, this badly affects the machine's productivity. Generally abrasive wear of plastic wires by the filler depends largely on the kind of filler used as well as on its particle size and form: the abrasive wear of wires is more marked with increasing particle size of the filler and with increasing number of knife edge-like projections on the surface of particles.
There are two alternatives to calcium carbonate used as filler for paper-making, namely precipitated calcium carbonate manufactured chemically by introducing carbon dioxide into milk of lime and heavy calcium carbonate manufactured by mechanically smashing lime stone and subsequent classification. Lime stone used as material of heavy calcium carbonate is roughly divided into two kinds. One kind is chalk obtained in Europe in large quantities. Not subjected to thermal metamorphism due to earth's magma activity, it is easily smashable, the resulting particle size is relatively uniform and there are scarcely any knife edge-like projections on the surface of particles. That the neutral paper-making process by the use of plastic wires has been widely adopted in Europe is because chalk of less abrasive effect on plastic wires has been readily available.
The other kind of lime stone is hard lime stone subjected to thermal metamorphism due to magma activity, so-called marble, and the greater part of lime stone used in Japan as material of heavy calcium carbonate belongs to this type. The particles of heavy calcium carbonate made from this marble type of lime stone are amorphous having many knife edges on the surface of particles, hence it is highly abrasive on plastic wires and is less suited as filler for neutral paper-making process by the use of plastic wires.
Precipitated calcium carbonate, which is manufactured chemically, is uniform in particle size, has fewer knife edges on the surface of particles and is relatively narrow in the width of particle size distribution. Compared with heavy calcium carbonate, therefore, it is less abrasive on plastic wires and is widely used as filler for rice paper and India paper. It is, however, inevitably dearer because of its manufacturing process, is low in yield because of its fine particle size and cannot impart enough strength to the paper made, hence it is less suited as filler for the neutral paper-making process and its use is limited to where it is used as substitute for expensive titanium oxide or as filler for papers of high added values such as information recording paper.
Heavy calcium carbonate, which is easier to manufacture than precipitated calcium carbonate, is cheaper but, when it is made from marble type lime stone obtained in Japan as mentioned above, the resulting particles are irregular, have many knife edges on the surface thereof and, when it is used as filler for neutral paper-making process, marked abrasive effect on plastic wires of the paper-making machine is inevitable. It is also possible to use heavy calcium carbonate extremely small in particle size with reduced abrasive effect on plastic wires as filler for neutral paper-making process. The industrial production of such fine particle size heavy calcium carbonate is, however, very small, its cost is even higher than that of talc, its yield in paper is lower with decreasing particle size and with that also decreases the strength of the paper, hence this can hardly be a good method.
For the above reasons, no heavy calcium carbonate has been available in Japan, which is similar to chalk of favorable behavior with regard to abrasive wear of plastic wires comparable with precipitated calcium carbonate. Hence, despite the well recognized merits of the neutral paper-making and the use of plastic wires, the neutral paper-making process by the use of plastic wires has not been adopted in earnest and the development of a method for mass-producing heavy calcium carbonate which is cheap, rated the same as or even lower than chalk in abrasive effect on plastic wires, favorable in yield and free of the effect of lowering the whiteness and strength of the paper made has been looked forward to, and extensive and intensive efforts have been made for this end.
In the specification of Laid-open Patent Application No. 144296/'83, for instance, it is claimed that the shape of particles of heavy calcium carbonate can be rounded by sand-milling to crumble off the knife edges formed in the surface. The method, however, calls for sand-milling a 30-85% aqueous suspension of heavy calcium carbonate once or even a plurality of times, which means a substantial increase in energy cost, and, moreover, the suspending agent used for improving the efficiency of sand-milling can possibly cause marked deterioration of the yield of the paper-making process.